Damping mechanisms of high-lying single-particle states in $^{91}$Nb

Decay by proton emission from high-lying states in 91Nb, populated in the 90Zr(alpha,t) reaction at Ealpha=180 MeV, has been investigated. Decay to the ground state and semidirect decay to the low-lying (2+,5-, and 3-) phonon states in 90Zr were observed. It was found that these phonon states play an important role in the damping process of the single-particle states. An optical-model coupled-channel approach was used successfully to describe the direct and semidirect parts of the decay.